Samples of baboon placentae have already been collected and preserved for study with the electron microscope. These specimens span the period from the time maternal blood comes into contact with trophoblastic tissue through the term of gestation. The ultrastructure of these specimens will be studied to establish the normal structure of the baboon placenta at various times during its development. Emphasis will be placed on defining the ultrastructural aspects of (1) the placental barrier, and (2) the decidua and the changes which take place during the course of gestation. This information will be used as baseline data for (1) future studies of the route of transport of materials of various molecular weights across the placental barrier and (2) comparison of the nature of the baboon decidua with that of the human in an attempt to explain many contradictory phenomena associated with these tissues, and establish similarity of function to the human.